The present invention relates to the drilling of oil and gas well. More particularly, the present invention relates to the processing of drilling fluid, such as drilling waste.
In the drilling of oil and gas wells, a drill bit is used to dig many thousands of feet into the earth's crust. Oil rigs typically employ a derrick that extends above the well drilling platform and that supports joint after joint of drill pipe connected end to end during the drilling operation. The drill pipe or “drill string” thus comprises a plurality of joints of pipe, each of which has an internal, longitudinally extending bore. The drill string bore carries drilling fluid, or “drilling mud”, from the well drilling platform through the drill string and to a drill bit supported at the lower, or distal, end of the drill string.
Drilling fluids can be any fluids and mixtures of fluids and solids, such as solid suspensions, mixtures, and emulsions of liquids, gases, and solids, used in operations to drill boreholes into the earth. Most drilling fluids are emulsions, which are a dispersion of one immiscible liquid into another. Chemicals such as emulsifiers can also be used to reduce the interfacial tension between the two liquids to achieve stability.
Two emulsion types are commonly used as drilling fluids. The first is oil-in-water, or direct, emulsion, known as “emulsion mud”. Emulsion muds are thus water-base drilling fluids that contain a dispersed oil or synthetic hydrocarbon as an internal phase. The second is water-in-oil, or invert, emulsion, known as an “invert emulsion mud”. Invert emulsion muds are thus oil-base muds that contain water as an internal phase.
The drilling fluid lubricates the drill bit and carries away small pieces of rock, or “drill cuttings”, such as shale generated by the drill bit as it digs deeper. The cuttings are carried in a return flow stream of drilling fluid through the well annulus and back to the well drilling platform at the earth's surface.
The drill cuttings are then separated from the reusable drilling fluid with commercially available separators known as “shale shakers”. Shale shakers operate by vibrating a wire-cloth screen while the drilling fluid flows on top of it. The liquid phase of the drilling fluid and solids smaller than the wire mesh pass through the screen, while larger solids are retained on the screen and eventually fall off the back of the device. Obviously, smaller openings in the screen clean more solids from the whole mud, but there is a corresponding decrease in flow rate per unit area of wire cloth. Some shale shakers are designed to filter coarse material from the drilling fluid while other shale shakers are designed to remove finer particles. Despite the use of shakers, a certain amount of drilling mud, which can contain hazardous oil, adheres to the drill cuttings. Other solids separators include mud cleaners and centrifuges that remove fine and ultrafine solids. Centrifuges comprise a rotating conical drum. Drilling fluid is fed into one end of the centrifuge and the separated solids are moved up the drum by a rotating scroll and exit at the other end.
After solids separation, the drill cuttings are disposed as drilling waste and the reusable drilling fluid is returned to a mud pit where it can be recycled into the well bore. In addition, over time the post-separation drilling fluid becomes too contaminated with fine solids particles that cannot be removed through typical separation techniques. Once the drilling fluid is no longer recyclable, it also becomes drilling waste and must be disposed.
After being run through the solids separators, the drilling waste comprising the drill cuttings and any residual drilling fluid is placed into holding containers. The drilling waste may also include the “recovered” drilling fluid that is no longer reusable in the well. The containers are then loaded onto either a truck or a boat for transportation to the processing facility. The drilling waste is then removed from the containers by washing the drilling waste out of the containers with large amounts of water.
Once emptied from the containers at the processing facility, the drilling waste is then processed using a thermal dryer. A thermal dryer typically consists of a sealed enclosure containing a rotating auger or other transferring means for moving the drilling waste through the enclosure. In the enclosure, the drilling waste is heated either directly or indirectly to evaporate any oil and water. The oil and water vapor are then recovered through condensation and are drawn off as liquids. The residual cleaned solid material is then pushed out of the dryer for bagging and disposal.